I completely agree -- and I could easily understand that someone would assume that's what is being asked, but would a child assume that (never being told that is the goal)? Should they have to assume that? I don't see why -- that seems to only "schoolify" them. Giving students incorrect answers only in order to conform them to the workings of the system is potentially damaging.
If they truly aim to teach math and not, say, engineering, then they should not be teaching students to assume constraints that aren't given in the problem.
It's also usually just a part of the course- like putting units on an answer in physics. You don't have to say so in every problem because you are taught from the start how to make your responses correct. I built a lot of content for a student math problem system and this is what the teacher wanted. 6/9 is an incorrect answer in elementary mathematics.